Everyday consumer products such as televisions and cellular telephones often contain integrated circuits that are configured to perform some type of electrical or processing function. These integrated circuits are fabricated on semiconductor wafers that may contain several copies of a particular integrated circuit. The wafer is processed to separate and produce individual integrated circuit “die” that then may be packaged into finished integrated circuits, often referred to as a “chip.” Functionality of an integrated circuit is generally verified by testing it. Such testing may be performed at the wafer level using a set of probe needles to contact each device (on-wafer measurements made during wafer sort) or may be done after each die has been packaged.
On-wafer testing is becoming increasingly important for radio frequency (RF) integrated circuit devices, such as monolithic microwave integrated circuit (MMIC) devices. It is sometimes more cost effective to test devices at the wafer level to screen out defective devices rather than perform the testing after the devices are packaged. For on-wafer testing, the performance of a device can generally be characterized by measuring certain parameters at the device terminals (ports) without regard to what is inside the device. Referring to FIG. 1, a RF device 10 may be modeled as a two port network having an input port 20 (generally port 1) and an output port 30 (generally port 2). Such a two port network may be characterized by any of several parameter sets including y-parameters (conductance), z-parameters (resistance), h-parameters (a mixture of conductance and resistance) or s-parameters (scattering). Each parameter set involves a set of four variables associated with the two-port model. For each parameter set, two of the variables represent the excitation of the network and the other two represent the response of the network to the excitation. Each of the two-port parameter sets describe the performance of the network. However, the variables and the parameters describing their relationships are different for each parameter set. At higher frequencies such as RF, s-parameters are generally easier to measure than other kinds of parameters.
For higher frequencies such as RF, the wavelength is comparable to the dimensions of the transmission line. For such frequencies, the representation of a network using a voltage and current approach like Y, Z, and H parameters becomes dependent on the point of measurement along the transmission line. This can be avoided by using S-Parameters to represent the network. A transmission line can be any pair of wires or conductors used to transmit the traveling waves from one point to another point, usually of controlled size and contained in a controlled dielectric material to create a controlled impedance. Thus, the s-parameters of a device under test (DUT) can be measured by a measurement system located at some distance from the DUT provided that the measurement system is connected to the DUT by coaxial cables, high quality strip lines or any other suitable low-loss transmission line.
FIG. 2 is a diagram of a two-port network 40 showing incident complex voltage waves 50, 60 (a1, a2) and reflected complex voltage waves 70, 80 (b1, b2) used in s-parameter definitions. As shown in FIG. 2, s-parameters are defined by complex voltage waves 50, 60 (having both a magnitude and phase component) incident on port 1 and port 2 and complex voltage waves 70, 80 reflected from port 1 and port 2 of the two-port network 40. That is, the s-parameters 90, 100, 110 and 120 (s11, s22, s21, and s12 respectively) relate the normalized traveling waves that are scattered or reflected when a device is inserted into a transmission line. The traveling waves 50, 60, 70 and 80 are normalized to the characteristic impedance ZO of the transmission line. S-parameters involve measurements with each port of the DUT stimulated in turn. For a two port DUT the microwave source 130 power is applied to each port. This is usually accomplished by using a microwave transfer switch to connect the source 130 to each port in turn. Due to the non-ideal nature of the switch, its effect is generally included in the measurement path during calibration. S-parameter testing generally involves the measurement of the DUT's four s-parameters 90, 100, 110 and 120 to verify that they are within design tolerances.
S-parameters are typically defined with the port not being stimulated terminated in a perfect load, ZO. For example, s11 90 (the input reflection coefficient) is equal to the ratio of the reflected wave 70 on port 1 to the input wave 50 on port 1 (b1/a1) with a perfect load 140 on port 2 (ZL=ZO). Use of a perfect load 140 makes the incident wave 60, a2, on port 2 zero. Thus, the accuracy of s-parameter measurements generally depends on how good a termination is applied to the port not being stimulated.
When a DUT is connected to the test ports of the measurement system, the measured s-parameters are only accurate when the measurement system is calibrated to minimize the effects of source and load impedance mismatch. This “systematic error” often does not vary over time and can be characterized during the calibration process and removed during the measurement process through a mathematical process called error correction. Measurement system calibration may also reduce other repeatable systematic errors caused by imperfections in the test equipment, cabling, load boards and RF probe cards including directivity and crosstalk errors related to signal leakage.
FIG. 3 is a block diagram illustrating a typical s-parameter wafer testing system 150. The measurement system test ports 160, 170 are connected to microstrip lines 180, 190 on a load board using coaxial cables 200, 210. The microstrip lines 180, 190 on the load board are used to connect the coaxial cables 200, 210 to wafer probe needles 220, 230 which in turn are used to connect to the DUT 240. A challenge in s-parameter measurements is to define where the measurement system 150 ends and the DUT 240 begins (see FIG. 3). This location is called the “measurement reference plane.” As shown in FIG. 3 there are multiple choices for where the measurement reference plane may be located in measurement system 150. For example, the measurement reference plane could be the defined as the Measurement System Test Port Reference Plane 250 located at the measurement system test ports 160, 170, the Coaxial Reference Plane 260 located at the ends of the coaxial cables 200, 210 or the On-Wafer Reference Plane 270 located at the ends of the probe needle tips 220, 230. However, choice of where the measurement reference plane is located is dependent on the availability of known reference standards used to calibrate the measurement system 150 that can be physically connected or inserted, preferably without the use of adaptors, at the measurement reference plane during the calibration process. This is because the calibration process involves measuring certain calibration standards of known characteristics and using these measurements to establish the measurement reference plane.
When known reference standards are available for insertion at the measurement reference plane, error contributions up to the measurement reference plane will be calibrated out. But any error contributions between the measurement reference plane and the DUT 240 become part of the measured DUT response. Ideally, the measurement reference plane should be the On-Wafer Reference Plane 270 located at the probe needle tips 220, 230 for on-wafer measurements so that just the DUT response is measured by the test system 150.
As discussed above, known reference standards are connected at the measurement reference plane during the calibration process. If adaptors are used to insert the reference standards at the measurement reference plane, the accuracy of the calibration may be degraded. This is a result of the calibration process using known calibration standards, i.e., standards that have been previously characterized, to determine the error correction as discussed below. Because adaptors are not ideal, use of them introduces additional errors that are not removed during the calibration process. For example, referring back to FIG. 3, choosing the Coaxial Reference Plane 260 as the measurement reference plane involves a set of standards that can be physically connected directly to the coaxial cables. Choosing the On-Wafer Reference Plane 270 as the measurement reference plane involves calibration standards that can be physically connected to the probe needle tips 220, 230.
There are two basic types of error correction: response calibration and vector error correction. Response calibration is a reduced error correction method, which is only used to de-embed the scalar transmission parameters |s12| and |s21| of the DUT. This is achieved by inserting a reference trace instead of the DUT 240. While response calibration is simple to perform, it removes only a few of the possible errors. Vector error correction is a more thorough method of error correction, but involves measuring phase as well as magnitude, and a set of calibration standards with known, precise electrical characteristics. The vector correction process characterizes the systematic errors by measuring known calibration standards. The difference between the measured and known responses of the standards is used to calculate an error model which is then used to remove the systematic errors from subsequent measurements.
There are several calibration methods available to do vector error correction when measuring the s-parameters of a two-port network. These include, but are not limited to Short-Open-Load-Thru (SOLT), Thru-Reflect-Line (TRL) and Line-Reflect-Line (LRL). For each of these calibration methods, specific, accurately know standards are measured during the calibration process. These calibration methods derive their names from the standards used during the calibration process.
For example, a calibration can be done at the coaxial ports 200, 210 of the measurement system 150 to remove the effects of the measurement system and any cables or adaptors that are a part of the calibration path. One of the most commonly used calibration methods for calibrating to the coaxial ports 200, 210 is the SOLT method because the characterized calibration standards are readily available. FIG. 4 shows a typical sequence of connection events for a two-port SOLT calibration. The SOLT calibration is done by making full S-Parameter measurements of the Open 280, Short 290, Load 300 and Thru 310 connected to port 1 and port 2. These measurements along with the known characteristics of the calibration standards allow the error correction for the forward direction, the source connected to port 1 with port 2 terminated, to be calculated. The error correction for the reverse direction, the source connected to port 2 with port 1 terminated, is calculated in a similar fashion.
The SOLT calibration method works well when the DUT 240 can be attached to the measurement system RF ports using the same connector types for which a precision calibration kit is available. However, if DUT 240 has non-standard connectors involving the use of adaptors or if non-standard probe cards are used to probe a device on a wafer, then it becomes more difficult to remove the effects of the measurement path from the device characteristics. This is a result of the measurement reference plane being established at the Coaxial Reference Plane 260 during the SOLT calibration procedure as shown in FIG. 3. Thus, any measurement errors caused by non-standard connectors, adaptors or probe cards inserted between the measurement reference plane and the DUT 240 are measured as part of the DUT response. That is, the measurement includes the effects (loss, phase shift, and mismatch) of the test fixture as well as the DUT response.
Additionally, the SOLT calibration method is not readily suited to calibrating s-parameter measurements made by automatic test equipment (ATE) testers during wafer sort because the calibration method involves a set of impedance standards that are not easily fabricated on the wafer. It can be difficult and costly to fabricate high quality SOLT standards on the wafer. FIG. 5 shows an example of SOLT calibration structures for calibrating ground-signal-ground (GSG) probes 320, 330 for on-wafer measurements. FIG. 5A shows the GSG probes connected to the Open structure 340 (probes in the air). FIG. 5B shows the GSG probes connected to the 50 ohm Load structure 350. FIG. 5C shows the GSG probes connected to the Short structure 360. FIG. 5D shows the GSG probes connected to the Thru structure 370.
None of these standards are ideal. For example the short structure 360 is not an ideal short, but rather behaves as an inductor at high frequencies. The open structure 340 is not an ideal open but rather behaves as a capacitor at high frequencies. In particular it is difficult to obtain a precise 50 ohm load structure 350 at high frequencies. Thus, such SOLT calibration standards are characterized prior to use. When a calibration is done using the characterized calibration standards, deviations from these known characteristics are treated as measurement system errors to be calibrated out.
Sometimes the SOLT calibration method is used during wafer sort by having the calibration standards fabricated on a separate wafer. This allows the set of reference standards to be characterized and the resistive load standard to be trimmed to its desired value, usually 50 ohms, prior to use of the standards. Prior to testing the DUT 240, the known standards are probed to calibrate the measurement system 150. This approach works well when the calibration standards are collocated with the test wafer containing the DUTs. However, this method becomes less desirable when space constraints involve swapping the test wafers and calibration standards during measurements. The SOLT calibration method is also impractical when the calibration standards are fabricated on the test wafer containing the DUTs. Here the calibration standards on each wafer need characterization to remove variations in the calibration standards from wafer to wafer. As discussed above, the precise 50 ohm load standard generally is trimmed to its desired value before use, which is impractical when the standards are fabricated on the same wafer as the DUTs.
Another method for calibrating s-parameter measurements of a two-port network is TRL. This calibration method uses thru, reflect and line calibration standards that can be implemented using transmission lines. The TRL calibration procedure involves making measurements with a Thru standard 390 connected to the test ports 400, 410, a Line standard 420 of unknown propagation constant but of known ZO connected to the test ports, and unknown high Reflect standards 430, 440 (open or short) connected to each of the test ports as shown in FIG. 6. The primary constraints when using this calibration technique are that the system impedance be equal to the characteristic impedance of the Line standard 420 and the reflect standards 430, 440 need to be the same on both test ports 400, 410. When a TRL calibration is performed, the reference plane is established at the middle of the Thru, which for a zero length Thru, is the DUT reference plane. However, in a wafer probing situation, the probe needles generally cannot be moved making it impossible to realize a zero length Thru 390 when the TRL standards are implemented on a wafer.
The TRL reference standards are more suitable than SOLT standards for fabrication on a non-coaxial media such as a semiconductor wafer because the TRL standards can be implemented with microstrips. FIG. 7 shows an example of a microstrip 450. The microstrip can be fabricated on a semiconductor wafer 460 by depositing a metal layer 470 on the surface of the wafer which is then etched to define the width of the microstrip. The impedance of the microstrip is determined by its geometry factor (w/t) and the relative permittivity constant of the semiconductor wafer (E) as is known to those skilled in the art.
FIG. 8 provides an example of TRL calibration standards implemented with microstrips. FIG. 8A shows a Thru structure 480 connected to the probes 490, 500. FIG. 8B shows a Reflect structure 510 connected to the probes 490, 500. FIG. 8C shows a Line structure 520 connected to the probes 490, 500. As shown in FIG. 8A, one of the standards used during the TRL calibration process is a Thru standard 480. This standard is generally implemented as a zero length line. Such a standard does not exist for on-wafer measurements because wafer probe needles 490, 500 often are rigid with a fixed spacing between the needles. As such, they cannot be directly connected to each other using a zero length thru standard. Rather, as shown in FIG. 8A, the Thru structure 480 can be implemented using microstrips with a non-zero length.
LRL is an alternative calibration method related to TRL. The calibration standards needed for LRL are two different line lengths, Line1 and Line2 standards 530, 540, and a Reflect standard 545 (usually an open or a short) as shown in FIG. 9. When the LRL calibration method is used, the reference plane is established by the reflect standard 545. Because the reflect standard 545 is used to establish the reference plane, it needs to be precisely characterized. This again presents problems when the standards are implemented on the same wafer as the DUT instead of on a fully characterized separate calibration wafer. Additionally, LRL is not suitable for calibrating probe cards with fixed spacing probe needles because LRL involves an undesirable change of probe-probe spacing during calibration to measure the longer line, usually Line2 standard 540.
What is needed is a calibration method that accounts for all the errors up to the device under test. That is, a method that establishes the reference plane at the DUT using calibration standards that are not precisely characterized. What is further needed is a calibration method that utilizes standards easily fabricated on a wafer and which can be used to calibrate fixed spacing probe cards. What is also needed is a method of calibration that supports the assignment of several calibration correction sets to a single RF measurement port to allow accurate measurement of several RF DUT pins that are connected to the RF port of the measurement system using a RF relay.